转载于 : http://blog.csdn.net/jcjc918/article/details/50395528

线程池介绍

线程池可以说是项目中经常会用到的组件，在这里假设读者都有一定的多线程基础，如果没有的话不妨在这里进行了解：POSIX 多线程基础。

线程池是什么？我的简单理解是有一组预先派生的线程，然后有一个管理员来管理和调度这些线程，你只需不断把需要完成的任务交给他，他就会调度线程的资源来帮你完成。

那么管理员是怎么做的呢？一种简单的方式就是，管理员管理一个任务的队列，如果收到新的任务，就把任务加到队列尾。每个线程盯着队列，如果队列非空，就去队列头拿一个任务来处理（每个任务只能被一个线程拿到），处理完了就继续去队列取任务。如果没有任务了，线程就休眠，直到任务队列不为空。如果这个管理员更聪明一点，他可能会在没有任务或任务少的时候减少线程的数量，任务处理不过来的时候增加线程的数量，这样就实现了资源的动态管理。

那么任务是什么呢？以后台服务器为例，每一个用户的请求就是一个任务，线程不断的在请求队列里取出请求，完成后继续处理下一个请求。

简单图示为： 

线程池有一个好处就是减少线程创建和销毁的时间，在任务处理时间比较短的时候这个好处非常显著，可以提升任务处理的效率。

线程池实现

这里介绍的是线程池的一个简单实现，在创建的时候预先派生指定数量的线程，然后去任务队列取添加进来的任务进行处理就好。

作者说之后会添加更多特性，我们作为学习之后就以这个版本为准就好了。

项目主页：threadpool

数据结构

主要有两个自定义的数据结构

threadpool_task_t

用于保存一个等待执行的任务。一个任务需要指明：要运行的对应函数及函数的参数。所以这里的 struct 里有函数指针和 void 指针。

typedef struct {void (*function)(void *); void *argument; } threadpool_task_t;

thread_pool_t

一个线程池的结构。因为是 C 语言，所以这里任务队列是用数组，并维护队列头和队列尾来实现。

struct threadpool_t {pthread_mutex_t lock;     /* 互斥锁 */pthread_cond_t notify;    /* 条件变量 */pthread_t *threads;       /* 线程数组的起始指针 */ threadpool_task_t *queue; /* 任务队列数组的起始指针 */ int thread_count; /* 线程数量 */ int queue_size; /* 任务队列长度 */ int head; /* 当前任务队列头 */ int tail; /* 当前任务队列尾 */ int count; /* 当前待运行的任务数 */ int shutdown; /* 线程池当前状态是否关闭 */ int started; /* 正在运行的线程数 */ };

函数

对外接口

• threadpool_t *threadpool_create(int thread_count, int queue_size, int flags); 创建线程池，用 thread_count 指定派生线程数，queue_size 指定任务队列长度，flags 为保留参数，未使用。
• int threadpool_add(threadpool_t *pool, void (*routine)(void *),void *arg, int flags); 添加需要执行的任务。第二个参数为对应函数指针，第三个为对应函数参数。flags 未使用。
• int threadpool_destroy(threadpool_t *pool, int flags); 销毁存在的线程池。flags 可以指定是立刻结束还是平和结束。立刻结束指不管任务队列是否为空，立刻结束。平和结束指等待任务队列的任务全部执行完后再结束，在这个过程中不可以添加新的任务。

内部辅助函数

• static void *threadpool_thread(void *threadpool); 线程池每个线程所执行的函数。
• int threadpool_free(threadpool_t *pool); 释放线程池所申请的内存资源。

线程池使用

编译

参考项目根目录下的 Makefile, 直接用 make 编译。

测试用例

项目提供了三个测试用例（见 threadpool/test/），我们可以以此来学习线程池的用法并测试是否正常工作。这里提供其中一个：

#define THREAD 32
#define QUEUE  256#include <stdio.h>
#include <pthread.h> #include <unistd.h> #include <assert.h> #include "threadpool.h" int tasks = 0, done = 0; pthread_mutex_t lock; void dummy_task(void *arg) { usleep(10000); pthread_mutex_lock(&lock); /* 记录成功完成的任务数 */ done++; pthread_mutex_unlock(&lock); } int main(int argc, char **argv) { threadpool_t *pool; /* 初始化互斥锁 */ pthread_mutex_init(&lock, NULL); /* 断言线程池创建成功 */ assert((pool = threadpool_create(THREAD, QUEUE, 0)) != NULL); fprintf(stderr, "Pool started with %d threads and " "queue size of %d\n", THREAD, QUEUE); /* 只要任务队列还没满，就一直添加 */ while(threadpool_add(pool, &dummy_task, NULL, 0) == 0) { pthread_mutex_lock(&lock); tasks++; pthread_mutex_unlock(&lock); } fprintf(stderr, "Added %d tasks\n", tasks); /* 不断检查任务数是否完成一半以上，没有则继续休眠 */ while((tasks / 2) > done) { usleep(10000); } /* 这时候销毁线程池,0 代表 immediate_shutdown */ assert(threadpool_destroy(pool, 0) == 0); fprintf(stderr, "Did %d tasks\n", done); return 0; }

源码注释

源码注释一并放在 github, 点我。

threadpool.h

/** Copyright (c) 2013, Mathias Brossard <mathias@brossard.org>.* All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions are* met:**  1. Redistributions of source code must retain the above copyright*     notice, this list of conditions and the following disclaimer.**  2. Redistributions in binary form must reproduce the above copyright*     notice, this list of conditions and the following disclaimer in the*     documentation and/or other materials provided with the distribution.** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT* HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.*/#ifndef _THREADPOOL_H_
#define _THREADPOOL_H_#ifdef __cplusplus /* 对于 C++ 编译器，指定用 C 的语法编译 */ extern "C" { #endif /** * @file threadpool.h * @brief Threadpool Header File */ /** * Increase this constants at your own risk * Large values might slow down your system */ #define MAX_THREADS 64 #define MAX_QUEUE 65536 /* 简化变量定义 */ typedef struct threadpool_t threadpool_t; /* 定义错误码 */ typedef enum { threadpool_invalid = -1, threadpool_lock_failure = -2, threadpool_queue_full = -3, threadpool_shutdown = -4, threadpool_thread_failure = -5 } threadpool_error_t; typedef enum { threadpool_graceful = 1 } threadpool_destroy_flags_t; /* 以下是线程池三个对外 API */ /** * @function threadpool_create * @brief Creates a threadpool_t object. * @param thread_count Number of worker threads. * @param queue_size Size of the queue. * @param flags Unused parameter. * @return a newly created thread pool or NULL */ /** * 创建线程池，有 thread_count 个线程，容纳 queue_size 个的任务队列，flags 参数没有使用 */ threadpool_t *threadpool_create(int thread_count, int queue_size, int flags); /** * @function threadpool_add * @brief add a new task in the queue of a thread pool * @param pool Thread pool to which add the task. * @param function Pointer to the function that will perform the task. * @param argument Argument to be passed to the function. * @param flags Unused parameter. * @return 0 if all goes well, negative values in case of error (@see * threadpool_error_t for codes). */ /** * 添加任务到线程池, pool 为线程池指针，routine 为函数指针， arg 为函数参数， flags 未使用 */ int threadpool_add(threadpool_t *pool, void (*routine)(void *), void *arg, int flags); /** * @function threadpool_destroy * @brief Stops and destroys a thread pool. * @param pool Thread pool to destroy. * @param flags Flags for shutdown * * Known values for flags are 0 (default) and threadpool_graceful in * which case the thread pool doesn't accept any new tasks but * processes all pending tasks before shutdown. */ /** * 销毁线程池，flags 可以用来指定关闭的方式 */ int threadpool_destroy(threadpool_t *pool, int flags); #ifdef __cplusplus } #endif #endif /* _THREADPOOL_H_ */

threadpool.c

/** Copyright (c) 2013, Mathias Brossard <mathias@brossard.org>.* All rights reserved.** Redistribution and use in source and binary forms, with or without* modification, are permitted provided that the following conditions are* met:**  1. Redistributions of source code must retain the above copyright*     notice, this list of conditions and the following disclaimer.**  2. Redistributions in binary form must reproduce the above copyright*     notice, this list of conditions and the following disclaimer in the*     documentation and/or other materials provided with the distribution.** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT* HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.*//*** @file threadpool.c* @brief Threadpool implementation file*/#include <stdlib.h>
#include <pthread.h> #include <unistd.h> #include "threadpool.h" /** * 线程池关闭的方式 */ typedef enum { immediate_shutdown = 1, graceful_shutdown = 2 } threadpool_shutdown_t; /** * @struct threadpool_task * @brief the work struct * * @var function Pointer to the function that will perform the task. * @var argument Argument to be passed to the function. */ /** * 线程池一个任务的定义 */ typedef struct { void (*function)(void *); void *argument; } threadpool_task_t; /** * @struct threadpool * @brief The threadpool struct * * @var notify Condition variable to notify worker threads. * @var threads Array containing worker threads ID. * @var thread_count Number of threads * @var queue Array containing the task queue. * @var queue_size Size of the task queue. * @var head Index of the first element. * @var tail Index of the next element. * @var count Number of pending tasks * @var shutdown Flag indicating if the pool is shutting down * @var started Number of started threads */ /** * 线程池的结构定义 * @var lock 用于内部工作的互斥锁 * @var notify 线程间通知的条件变量 * @var threads 线程数组，这里用指针来表示，数组名 = 首元素指针 * @var thread_count 线程数量 * @var queue 存储任务的数组，即任务队列 * @var queue_size 任务队列大小 * @var head 任务队列中首个任务位置（注：任务队列中所有任务都是未开始运行的） * @var tail 任务队列中最后一个任务的下一个位置（注：队列以数组存储，head 和 tail 指示队列位置） * @var count 任务队列里的任务数量，即等待运行的任务数 * @var shutdown 表示线程池是否关闭 * @var started 开始的线程数 */ struct threadpool_t { pthread_mutex_t lock; pthread_cond_t notify; pthread_t *threads; threadpool_task_t *queue; int thread_count; int queue_size; int head; int tail; int count; int shutdown; int started; }; /** * @function void *threadpool_thread(void *threadpool) * @brief the worker thread * @param threadpool the pool which own the thread */ /** * 线程池里每个线程在跑的函数 * 声明 static 应该只为了使函数只在本文件内有效 */ static void *threadpool_thread(void *threadpool); int threadpool_free(threadpool_t *pool); threadpool_t *threadpool_create(int thread_count, int queue_size, int flags) { if(thread_count <= 0 || thread_count > MAX_THREADS || queue_size <= 0 || queue_size > MAX_QUEUE) { return NULL; } threadpool_t *pool; int i; /* 申请内存创建内存池对象 */ if((pool = (threadpool_t *)malloc(sizeof(threadpool_t))) == NULL) { goto err; } /* Initialize */ pool->thread_count = 0; pool->queue_size = queue_size; pool->head = pool->tail = pool->count = 0; pool->shutdown = pool->started = 0; /* Allocate thread and task queue */ /* 申请线程数组和任务队列所需的内存 */ pool->threads = (pthread_t *)malloc(sizeof(pthread_t) * thread_count); pool->queue = (threadpool_task_t *)malloc (sizeof(threadpool_task_t) * queue_size); /* Initialize mutex and conditional variable first */ /* 初始化互斥锁和条件变量 */ if((pthread_mutex_init(&(pool->lock), NULL) != 0) || (pthread_cond_init(&(pool->notify), NULL) != 0) || (pool->threads == NULL) || (pool->queue == NULL)) { goto err; } /* Start worker threads */ /* 创建指定数量的线程开始运行 */ for(i = 0; i < thread_count; i++) { if(pthread_create(&(pool->threads[i]), NULL, threadpool_thread, (void*)pool) != 0) { threadpool_destroy(pool, 0); return NULL; } pool->thread_count++; pool->started++; } return pool; err: if(pool) { threadpool_free(pool); } return NULL; } int threadpool_add(threadpool_t *pool, void (*function)(void *), void *argument, int flags) { int err = 0; int next; if(pool == NULL || function == NULL) { return threadpool_invalid; } /* 必须先取得互斥锁所有权 */ if(pthread_mutex_lock(&(pool->lock)) != 0) { return threadpool_lock_failure; } /* 计算下一个可以存储 task 的位置 */ next = pool->tail + 1; next = (next == pool->queue_size) ? 0 : next; do { /* Are we full ? */ /* 检查是否任务队列满 */ if(pool->count == pool->queue_size) { err = threadpool_queue_full; break; } /* Are we shutting down ? */ /* 检查当前线程池状态是否关闭 */ if(pool->shutdown) { err = threadpool_shutdown; break; } /* Add task to queue */ /* 在 tail 的位置放置函数指针和参数，添加到任务队列 */ pool->queue[pool->tail].function = function; pool->queue[pool->tail].argument = argument; /* 更新 tail 和 count */ pool->tail = next; pool->count += 1; /* pthread_cond_broadcast */ /* * 发出 signal,表示有 task 被添加进来了 * 如果由因为任务队列空阻塞的线程，此时会有一个被唤醒 * 如果没有则什么都不做 */ if(pthread_cond_signal(&(pool->notify)) != 0) { err = threadpool_lock_failure; break; } /* * 这里用的是 do { ... } while(0) 结构 * 保证过程最多被执行一次，但在中间方便因为异常而跳出执行块 */ } while(0); /* 释放互斥锁资源 */ if(pthread_mutex_unlock(&pool->lock) != 0) { err = threadpool_lock_failure; } return err; } int threadpool_destroy(threadpool_t *pool, int flags) { int i, err = 0; if(pool == NULL) { return threadpool_invalid; } /* 取得互斥锁资源 */ if(pthread_mutex_lock(&(pool->lock)) != 0) { return threadpool_lock_failure; } do { /* Already shutting down */ /* 判断是否已在其他地方关闭 */ if(pool->shutdown) { err = threadpool_shutdown; break; } /* 获取指定的关闭方式 */ pool->shutdown = (flags & threadpool_graceful) ? graceful_shutdown : immediate_shutdown; /* Wake up all worker threads */ /* 唤醒所有因条件变量阻塞的线程，并释放互斥锁 */ if((pthread_cond_broadcast(&(pool->notify)) != 0) || (pthread_mutex_unlock(&(pool->lock)) != 0)) { err = threadpool_lock_failure; break; } /* Join all worker thread */ /* 等待所有线程结束 */ for(i = 0; i < pool->thread_count; i++) { if(pthread_join(pool->threads[i], NULL) != 0) { err = threadpool_thread_failure; } } /* 同样是 do{...} while(0) 结构*/ } while(0); /* Only if everything went well do we deallocate the pool */ if(!err) { /* 释放内存资源 */ threadpool_free(pool); } return err; } int threadpool_free(threadpool_t *pool) { if(pool == NULL || pool->started > 0) { return -1; } /* Did we manage to allocate ? */ /* 释放线程 任务队列 互斥锁 条件变量 线程池所占内存资源 */ if(pool->threads) { free(pool->threads); free(pool->queue); /* Because we allocate pool->threads after initializing the mutex and condition variable, we're sure they're initialized. Let's lock the mutex just in case. */ pthread_mutex_lock(&(pool->lock)); pthread_mutex_destroy(&(pool->lock)); pthread_cond_destroy(&(pool->notify)); } free(pool); return 0; } static void *threadpool_thread(void *threadpool) { threadpool_t *pool = (threadpool_t *)threadpool; threadpool_task_t task; for(;;) { /* Lock must be taken to wait on conditional variable */ /* 取得互斥锁资源 */ pthread_mutex_lock(&(pool->lock)); /* Wait on condition variable, check for spurious wakeups. When returning from pthread_cond_wait(), we own the lock. */ /* 用 while 是为了在唤醒时重新检查条件 */ while((pool->count == 0) && (!pool->shutdown)) { /* 任务队列为空，且线程池没有关闭时阻塞在这里 */ pthread_cond_wait(&(pool->notify), &(pool->lock)); } /* 关闭的处理 */ if((pool->shutdown == immediate_shutdown) || ((pool->shutdown == graceful_shutdown) && (pool->count == 0))) { break; } /* Grab our task */ /* 取得任务队列的第一个任务 */ task.function = pool->queue[pool->head].function; task.argument = pool->queue[pool->head].argument; /* 更新 head 和 count */ pool->head += 1; pool->head = (pool->head == pool->queue_size) ? 0 : pool->head; pool->count -= 1; /* Unlock */ /* 释放互斥锁 */ pthread_mutex_unlock(&(pool->lock)); /* Get to work */ /* 开始运行任务 */ (*(task.function))(task.argument); /* 这里一个任务运行结束 */ } /* 线程将结束，更新运行线程数 */ pool->started--; pthread_mutex_unlock(&(pool->lock)); pthread_exit(NULL); return(NULL); }